No estoy 100% seguro, pero he leido que "that" puede ser omitida cuando despues de ella hay un pronombre personal. Im not sure 100%, but I have read that "that" may be omitted when after there is a personal pronoun you can write It Is the best I do or It Is the best that I do both are correct I hope to explain me

This is a great observation. Let me explain more about this. In English there are two kinds of relative clauses: 1. Subject relative clauses: A doctor is a person WHO/THAT HELPS SICK PEOPLE. I'm looking for an apartment THAT/WHICH HAS TWO BEDROOMS. 2. Object relative clauses: Tom is the person WHO/THAT YOU MET LAST WEEK. That isn't the kind of film THAT/WHICH I LIKE. You can probably notice the difference between subject and object relative clauses. In subject relative clauses, the noun before who/which/that/who does the action ... the doctor helps sick people, the apartment has two bedrooms, etc. In object relative clauses, the noun before who/which/that/who receives the action ... you met the person, I like the film. In an OBJECT relative clause, you do not need to use a relative pronoun (who/which/that/who). You can use it or you can not use it; it's optional. Tom is the person you met last week. That isn't the kind of film I like. So you are correct that people omit 'that' before I; but remember that it can also be omitted in any object relative clause. Also please remember that a relative pronoun is mandatory (not optional) in subject relative clauses.